The present invention relates to a method of bonding alternating conductive and glass layers. More particularly, the method pertains to anodic bonding of stacks of alternating conductive and glass layers, where the conductive layer is a metal or semiconductor. The invention has applicability, among other areas, in the formation of such stacks for microcolumns in electron optics, including electron microscopes and lithography apparatus, in the formation of micro electromechanical structures (MEMS), and also in micro opto-electromechanical structures (MOEMS).
Stacks of alternating layers of conductive material and glass find use in a number of practical applications such as in electron optics and micro electromechanical structures. Anodic bonding has been one of the techniques used to bond the conductive layer to the glass layer. In some instances, a semiconductor material such as silicon is used as the conductive layer, and the glass layer is a borosilicate glass, such as PYREX(copyright) (Corning Glass, Corning, N.Y.) or BOROFLOATO(copyright) (Schott Glass Technologies, New York, N.Y.). In the alternative, the glass layer may be a lithium aluminosilicate -xcex2-quartz glass-ceramic such as Prototype PS-100 available from HOYA Co., Tokyo, Japan. The advantage of this latter glass is that anodic bonding may be performed at a temperature below about 180xc2x0 C.
A detailed description of use of the HOYA Co. Lithium aluminosilicate -xcex2-quartz glass-ceramic glass is provided in a publication by Shuichi Shoji et al. entitled: xe2x80x9cAnodic Bonding Below 180xc2x0 C. For Packaging And Assembling Of MEMS Using Aluminosilicate-xcex2-quartz Glass-Ceramicxe2x80x9d, available form IEEE as document 0-7803-3744-1/97, the subject matter of which is hereby incorporated by reference in its entirety. In particular the bonding of Prototype PS-100 glass-ceramic pieces 370 xcexcm thick to silicon wafers was achieved using anodic bonding at a temperature ranging from about 140xc2x0 C. to about 180xc2x0 C., at an applied DC voltage ranging from about 300 V to about 700 V, over a time period of about 10 minutes or less. A comparison is made for bonding the Prototype PS-100 glass relative to #7740 Corning PYREX(copyright) glass and relative to #SD-2 HOYA Bonding Glass. In all cases, a single layer of glass is bonded to a layer of silicon.
One conventional approach to anodic bonding is shown in FIG. 1. In this Figure, conductive layers (silicon layers, by way of example) 108, 110, 112, and 114 are alternated with electrically insulating layers (borosilicate glass, by way of example) 107, 109, and 111. The stack 100 of alternating silicon and glass layers is placed upon a hotplate 106, which provides both a source of heat input and electrical grounding. Electrical contact 102 is contacted to uppermost silicon layer 108, while electrical contact 104 is contacted to the hotplate 106. Silicon layer 108 acts as the upper electrode, while silicon layer 114/hotplate 106 acts as the lower electrode. Heat is applied to the hotplate 106 and a voltage is applied between the electrodes 108 and 114/106, through all of the layers to be bonded. The heated glass acts as an electrochemcial cell and permits the transfer of current through the borosilicate glass layers 107, 109, and 111. The application of the voltage causes ionized sodium and oxygen to move within the glass and promotes bonding of silicon layer surfaces to glass layer surfaces.
Looking at the process in a little more detail, anodic bonding has been accomplished using either DC voltage or AC voltage. Accordingly, for purposes of the following description, the voltage source in FIG. 1 is shown in conceptual, rather than structural form.
In the DC voltage technique, a negative DC potential is applied between electrodes 108 and 114/106, followed by application of reverse polarity DC potential between the electrodes 108 and 114/106.
When, for example, electrode 114/106 is at ground potential, and electrode 108 is at a negative potential, oxygen ions travel toward surface 132 of glass layer 107; surface 134 of glass layer 109; and, surface 136 of glass layer 111. This enables the covalent bonding of oxygen to silicon at surface 132 between glass layer 107 and silicon layer 110; at surface 134, between glass layer 109 and silicon layer 112; and, at surface 136, between glass layer 111 and silicon layer 114. Simultaneously, application of the DC voltage in this manner causes sodium ions that are part of the glass layers to move toward the opposite surface of each glass electrochemical cell. For example, sodium ions move toward surface 131 of glass layer 107; surface 133 of glass layer 109; and, surface 135 of glasslayer 111.
The series connection of the electrochemical cells creates a potential gradient over the entire stack. Since current flows throughout the stack 100, from top electrode 108 to bottom electrode 114/106, each silicon layer acting as an electrode, the electrode surface includes the entire major surface of each of the stacked silicon layers.
After application of the DC potential in this fashion, in the next step in the anodic bonding process, the voltage is reversed, such that electrode 114/106 is at a negative potential, and electrode 108 is at ground. This permits oxygen ions to move within glass layer 107 toward surface 131; within glass layer 109 toward surface 133; and within glass layer 111 toward surface 135. However, the covalent bonding of the oxygen to the silicon at surfaces 131, 133, and 135 is weaker due to the presence of the sodium compounds 120, 122, and 124, respectively, which form due to the movement of sodium ions toward these surfaces during the bonding process. Simultaneously with the covalent bonding of surfaces 131, 133, and 135, sodium compounds 126, 128, and 130 form at surfaces 132, 134, and 136 of glass layers 107, 109, and 111, respectively, weakening the bond between these glass surfaces and the mating silicon surfaces.
In view of the weakened bonds formed at silicon surfaces 131, 133 and 135, as described above, an AC voltage anodic bonding technique was devised. By applying an AC voltage, voltage polarities are reversed continuously, thus achieving bonding between all adjoining surfaces of consecutive layers. By applying AC voltage, the concentration of sodium at each interface during bonding is gradually increased during the bonding period. This means the amount of sodium contamination is lower at the beginning of the bonding process, which better facilitates bonding. However, by the end of the process the sodium contamination has reached a significant level, and the overall bond strength between the alternating layers may not be adequate for some applications.
In view of the foregoing deficiencies, it would be desirable to be able to bond semiconductor and glass layers anodically, without the concentration of sodium and sodium compounds at the interface of bonding layers.
We have developed a method of anodic bonding which directs cations to a location within a bonding structure which is away from critical bonding surfaces. This prevents the formation of compounds comprising the cations at the critical bonding surfaces. The anodic bonding electrode contacts are made in a manner which concentrates the cations and compounds thereof in a portion of the bonded structure which can be removed, or cleaned to remove the compounds from the structure. A device formed from the bonded structure contains minimal, if any, of the cation-comprising compounds which weaken bond strength within the structure. In the alternative, the cations and compounds thereof are directed to a portion of the bonding structure which does not affect the function of a device which includes the bonded structure.